1. Field of the Invention
This invention relates to a method for forming a thin layer of material on the surface of a semiconductor substrate and an apparatus for the same. In particular, this invention relates to a novel method for forming a thin layer of material on the surface of a semiconductor substrate after the substrate surface ha been cleared of unwanted organic contaminants, metal contaminants and damaged surface, and without exposing the cleaned substrate surface to the ambient air, and an apparatus for carrying out the method.
2. Description of the Background Art
Performance characteristics of electronics devices are readily and adversely affected by contaminants introduced on purpose or by accident in the course of the manufacturing process. In order to avoid any introduction of contaminants into the electronics products as much as possible, it is necessary to keep the whole manufacturing environment at a maximum possible degree of cleanness. In this connection, highly advanced cleaning and purifying technologies are employed to produce desired starting materials and processing environments for the manufacture of the devices.
The manufacturing process for semiconductor devices is roughly divided into a thin film formation and a circuit pattern formation. The process for forming thin films or layers of material on the semiconductor substrate is further divided into many subprocesses depending on the material of which the thin films are made and techniques to be used to form them. Various cleaning technologies for each of the subprocesses or to be shared by some of the subprocesses have been developed into practical use. Important and essential to all these subprocesses is the pretreatment of the semiconductor substrate which is performed prior to forming the thin films on the substrate.
In the pretreatment process, the semiconductor substrate is cleaned with water, acid or alkalis or it is subjected to chemical oxidation or treatment with diluted hydrofluoric acid solution for the removal of grease heavy metals, naturally grown oxide film and the like. These solution cleaning techniques are widely used in the industry but are disadvantageous in that the cleaned semiconductor substrate is unavoidably exposed to the surrounding air before it is coated with thin films in a subsequent process. The exposure to the air causes a thin oxide film to grown on the substrate surface, especially when the substrate contains active semiconductor substance or it has exposed metal portions thereon. For this reason, the substrate cleaning with solutions falls short of complete cleaning of the substrate surface although it is effective to remove heavy metal and organic contaminants.
The growth of a natural oxide film on the semiconductor substrate has an adverse effect on the quality of various thin films to be formed on the substrate in subsequent steps. The thin films provided on the semiconductor substrate includes epitaxially grown layers, layers of high melting point metals or polysilicide layers, electrical interconnection layers, ultra-thin insulating layers. The formation of these layers on the semiconductor substrate is gaining a growing importance as the integration of semiconductor devices advances.
In order to have a brief background understanding, references is made to FIG. 11 which illustrates a prior-art apparatus for sputter-forming thin layer on a semiconductor substrate. As shown, the apparatus includes a chamber 1 which are divided by partition walls into a loading compartment 3, and an etching compartment 4, a depositing compartment 5 and an unloading compartment 6. The partition walls are provided with passage openings to be covered with lock valves 2a, 2b and 2c, respectively.
The loading compartment 3 is equipped with an inlet 7a for nitrogen gas, and an exhaust outlet 8. Provided within the loading compartment 3 is a belt conveyer 9a as transport means.
The etching compartment 4 is equipped with an inlet 10a for argon gas and an exhaust outlet 8b. Housed within the etching compartment 4 are a belt conveyer 9b and a substrate support 11a with a tray 12 for accommodating a semiconductor substrate attached thereto. The substrate support 11a is coupled to a radio frequency source 14 through a matching circuit 15, and is electrically insulated from the chamber 1 by an insulation 16a.
The depositing compartment 5 is also provided with an inlet 10a for argon gas and an exhaust outlet 8c. Housed within the depositing compartment are a belt conveyer 9c, a substrate support 11b and a target 17. The target 17 is connected to a DC source 18 which applies a high potential voltage across the target 17 and the argon gas atmosphere within the depositing compartment. An insulation 16b electrically separates the substrate support 11b from the chamber 1, and another insulation 16c provides an electrical insulation between the target 17 and the chamber 1.
The unloading compartment 6 is equipped with an inlet 7b for nitrogen gas, and an exhaust outlet 8d. It also houses a conveyer belt 9d.
With the apparatus, a thin layer of material is formed on the semiconductor substrate as follows.
All of the loading compartment 3, etching compartment 4, depositing compartment 5 and unloading compartment 6 in the chamber 1 are first kept in a high-intensity vacuum state. Then nitrogen gas is introduced into the loading compartment 3 via the gas inlet 7a thereby to attain the atmospheric pressure within the loading compartment. A plurality of silicon substrates are carried into the loading compartment through an entry opening (not shown), after which the entrance opening is closed with a cover lid (not shown). It should be noted that the silicon substrate has been cleaned with a solution but has already an oxide film grown on its surface. The loading compartment 3 is exhausted through the outlet 8a using a vacuum pump (not shown) thereby to produce a high-intensity vacuum within the compartment. With the loading compartment in this high vacuum state, the lock valve 2a is swung open and the tray 12 carrying the silicon substrates is transferred through the passage opening in the partition into the neighboring etching compartment 4 by means of the belt conveyer 9a. Thereafter, the lock valve 2a is moved back to its passage closing position. The operations of both the belt conveyer 9a and the movable lock valve 2a are controlled by suitable externally provided control unit such as a switching unit. As stated above, the etching compartment 4 is kept at a high vacuum of 10.sup.-7 -10.sup.-8 Torr. With the silicon substrate 13 on the tray 12 placed on the support 11a, an argon gas is introduced into the deposit compartment at 10.sup.-3 -10.sup.-1 Torr, followed by switching the RF source 14 into operation. As the RF source 14 is turned on, it applies a high potential of several hundred to several thousand volts across the silicon substrate 13 and the argon atmosphere within the compartment to produce an argon plasma. Excited argon ions in the plasma bombard the silicon substrate kept at a negative potential. The bombarding argon ions act to sputter-etch the naturally grown oxide film of the surfaces of the silicon substrate 13. Upon the complete removal of the oxide film, the supply of the argon gas and the high potential are both discontinued. The remaining argon gas is expelled out of the etching compartment 4 to keep the compartment in a high vacuum state. Under the conditions, the lock valve 2b is swung open to allow the substrate carrying tray 12 to be transported into the deposit compartment 5 by the belt conveyer 9b. At about the same time, a second tray with a plurality of silicon substrate is carried into the etching compartment 4. The deposit compartment 5 has also been maintained at a high vacuum of 10.sup.-7 -10.sup.-8 Torr. Now argon gas is introduced into deposit compartment at 10.sup.-3 -10.sup.-1 Torr through the inlet 10b. Then the DC power supply 18 is turned on to apply a potential of several hundred to several thousand volts across the target 17 and the argon gas in the compartment, thereby creating argon plasma. Highly excited argon ions in the plasma bombard the target 17 which is kept at a negative potential and strike atoms off the surface of the target. As a result of the process, a thin layer having a uniform and homogeneous quality is deposited on the surface of the silicon substrate. As the deposited layer grows to a desired thickness, the supply of the argon gas as well as the high potential voltage is discontinued. Then, the deposit compartment 5 is again exhausted to a high vacuum state.
The lock valve 2c is moved to an open position and the belt conveyer 9c carries the tray 12 with the silicon substrate 13 into the unloading compartment 6. Thereafter, the lock valve 2c is swung back to the closed position. At about the same time, the second tray is brought into the deposit compartment 5, while a third tray into the etching compartment 4. In this manner, the trays with silicon substrates are moved through the etching and deposit compartment into the unloading compartment 6 one after another. In the unloading compartment 6, the silicon substrate 13 on each tray are loaded off into a container (not shown). When the silicon substrates on all of the trays have been loaded into the container, the unloading compartment 6 is restored to the atmospheric pressure and the substrate-filled container is carried out of the unloading compartment to a next process location.
As has been stated hereinabove, with the arrangement of the conventional apparatus, it is necessary to apply a high potential voltage between the silicon substrate and the argon gas in order to remove the naturally grown oxide film from the substrate surface. However, the argon plasma generated by the high potential tends to cause damage on the surface of the silicon substrates.
It has also been proposed to gas-etch the natural oxide film on a semiconductor substrate through high temperature hydrogen reduction technique, followed by forming thin layers of desired metals. But the hydrogen reduction process exposes the semiconductor substrate to elevated temperatures normally above 1,000.degree. C., causing thermal fusion at the PN junction. This in turn limits the application of the hydrogen reduction technique.